This invention relates to a multi-layer heat-sealable structure which has exceptionally low film to film coefficient of friction over a wide temperature range and which, when used to wrap packages secured by a heat-seal layer or layers, avoids package to package sticking in any heat seal region.
Highly crystalline polypropylene film is an excellent packaging material, but it has high film to film coefficient of friction which makes it difficult to utilize in automatic packaging equipment. This material in film form has a tendency to stick at heat seal surfaces when adjacent heat seal regions contact shortly after sealing. Some years ago cigarette packaging machines had a top speed of approximately 140 packs per minute. Present day machines run at from 300-400 packs per minute, which is equivalent to approximately 160 ft. of film per minute. This speed makes the handleability or machineability of film, i.e. the ability of the film to be machine utilized, very critical. Film breakage or partial tearing and package to package sticking causes extremely expensive machine down time which cannot be tolerated.
Certain innovations have improved on the surface friction characteristics of highly crystalline polypropylene in film form. For example, in U.S. Pat. No. 3,176,021, incorporated herein by reference in its entirety, it is taught to include minor quantities of fatty acid amides into the polypropylene. In order to obtain the benefits taught by this patent, certain limitations must be observed. The film must be formed from a melt, extruded at a temperature between about 400.degree.-550.degree. F. In addition, the amide must be present in from 0.005 to about 2.0 weight percent of the polypropylene and it must be present along with from about 0.1 to about 4.0 weight percent polyethylene. Under these conditions and limitations the resulting polypropylene film will have a static coefficient of friction no higher than 0.6 which is significantly higher than present day requirements. In addition, such a film does not have the high stereoregularity required by present day packaging demands. Further, it has been found that once said film has been subjected to the high temperatures which will be experienced in practice, particularly during heat sealing, i.e., approaching 270.degree. F., the coefficient of friction increases significantly and is nowhere near the present day requirements of 0.25.
In U.S. Pat. No. 3,399,156, incorporated herein by reference in its entirety, it is taught that the inclusion of the combination of silicone oil and finely divided silica into a polypropylene film produces a film which will have a coefficient of friction of from 0.45 to 0.46 and be suitable for use in bread-wrapping machines. This is not a low enough co-efficient of friction for use in present day, high speed automatic packaging machinery.
It has been found that to a certain extent high crystallinity in polypropylene impedes facile migration of additives to the surface of such a polymer in film form. This problem has been addressed in U.S. Pat. No. 4,419,410, incorporated herein by reference in its entirety. This application teaches that by the coextrusion of a skin layer of polypropylene of comparatively low stereoregularity with a core layer of polypropylene of comparatively high stereoregularity, where this core layer contains a surface modifying agent such as an amide of a fatty acid, the amide will bloom to the surface of the comparatively low stereoregularity polymer with comparative ease.
In spite of these teachings, it remained until U.S. Pat. No. 4,419,411, incorporated herein by reference in its entirety, to significantly advance the art of making multil-layer polypropylene films suitable for most high speed packaging operations. This patent teaches a multi-layer polypropylene film structure having surface modifying agents comprising finely divided silica, silicone oil and an amide of a water insoluble mono-carboxylic acid having from about 8 to about 24 carbon atoms on a surface thereof.
Notwithstanding the general excellence of this film, it is found that when used in high speed cigarette package wrapping machines, there is a tendency for the film surfaces of contacting packages to stick together, particularly in the areas where heat sealing has occurred.
It is an object of the present invention to provide a multi-layer film structure having at least one surface layer having an extremely low coefficient of friction over a wide temperature range, said film also having reduced tendency of the film surfaces to stick together when used in high speed packaging machinery.
It is another object to provide a process for preparing this multi-layer structure.